1. Technical Field
This invention relates generally to a method for reducing hydrocarbon emissions in direct injected spark ignition engines, and more particularly to such a method using a catalytic oxidative coating on a piston for post-combustion oxidation of hydrocarbons during an expansion stroke of the engine.
2. Background Art
There is world-wide interest directed to the development of internal combustion engines with fuel injection systems which introduce fuel directly into a combustion chamber. Direct injection is advantageous for several reasons, including improved efficiency and reduced fuel consumption. However, direct injected gasoline engines produce significantly higher levels of hydrocarbon (HC) emissions than comparable port-fuel injected engines. One of the main mechanisms responsible for high HC emissions in direct injected gasoline engines has been found to be attributable to the deposition of liquid fuel on the surfaces of the combustion chamber (i.e., the piston top, cylinder liner, etc.). This phenomenon is generally referred to as combustion chamber wall wetting, and is most predominant when fuel is injected late during the compression stroke. Most current directed injected, spark ignition (DISI) engines are designed so that some wetting of the piston top occurs during light-load operation. Recent research, as evidenced in "The Effect of In-Cylinder Wall Wetting Location on the HC Emissions from SI Engines", Rudolf Stangimaier, et al. 1999 SAE Technical Paper Series, indicates that piston top wetting contributes significantly to the hydrocarbon emissions from DISI engines.
Catalytic coatings within combustion chambers have been used for ignition of the fuel/air mixture as a result of contact with the catalytic material present in the combustion chamber. This form of catalytic-ignition is described in U.S. Pat. Nos. 4,773,368; 4,811,707; 4,819,595; and 4,905,658, all issued to William C. Pfefferle. More recently, U.S. Pat. No. 5,806,483, issued Sep. 15, 1998, to Mitarai, et al describes a combustion method for preventing the occurrence of knock during engine operation by providing a two-stage combustion process which is substantially different from the normal spark-ignition combustion process. Cracking of carbon-to-carbon bonds in heavier fuel molecules immediately prior to combustion is described in U.S. Pat. No. 4,530,340, granted Jul. 23, 1985, to Millard C. Totman. Totman uses a catalytic coating within the combustion chamber to promote the pre-combustion cracking of the heavier fuel molecules. Earlier, U.S. Pat. No. 2,978,360 was issued to Samuel W. Bradstreet, et al on Apr. 4, 1961 for several specific catalyst formulations and methods for generating and applying catalytic coatings to combustion chamber surfaces. The coatings are provided for the purpose of maximizing the combustion process itself, and as a result of the more efficient combustion, reduced overall emissions.
The present invention is directed to overcoming the problem of piston top wetting and the resultant contribution to hydrocarbon emissions. It is desirable to have a direct injected spark ignition engine that promotes a catalytic reaction at the top of the piston whereby a higher fraction of the fuel which impinges on the piston top is oxidized during the expansion stroke. It is also desirable to have such an engine wherein the detrimental effects of wall wetting on hydrocarbon emissions are reduced.